X-ray target.



W. D. COOLIDGE.

X-RAY TARGET.

APPLICATION FILED APR. 10. 1915.

1 9 1 fi fi m o Patented Sept. 14, 1915;

lnvntovr William DQCooHolge, y WM Hi S Attorney IIWIM @TATTEd AEWT UI I@M.

WILLIAM I). COOLIDGE, 0F SCHENEOTAZD'Y, NEW YORK, ASSIGNOR IO GENERALELECTRIC COMPANY,A CORPORATION OF NEW YORK.

X-RAY TARGET.

Specification of Letters Patent.

Patented Sept. 1d, 1915.

Application filed April 10, 1915. Serial No. 20,409.

To all whom it may concern Be it known that I, WILLrAM D. COOLIDGE, acitizen of the United States, residing at Schenectady, county ofSchenectady, State of New York, have invented certain new and usefulImprovements in X-Ray Targets, of which the following is aspecification.

In the operation of X-ray tubes, a stream of cathode rays, consisting ofelectrons traveling at high velocity, impinges on the surface of a metalbody known as the target, which commonly also serves as anode. When thecathode rays strike the target surface, part of their energy isconverted into X-rays but the larger part is'converted into heat. It isdesirable to have the cathode rays impinge on a highly refractory metaland for this purpose tungsten is particularly suitable. The tungstendisk serving as the target proper has in some cases been embedded ina-mass of copper to increase the heat storage capacity of the anode andto carry heat away from the focal spot. When an electrode consisting inpart of copper is used in X-ray tubes operating with a substantiallypure electron discharge, such as described in the Physical Review forDecember 1913, care must be exercised not to raise the temperature farabove redness, otherwise gas is evolved and copper is vaporized. A pureelectron discharge can take place only when the gas pressure is notgreater than a given minimum pressure, the particular value of whichdepends on the voltage of the discharge. As the natural cooling of abody in a vacuum at temperatures below redness proceeds very slowly, apure electron discharge X-ray tube equipped with a copperbacked targetis subject to undesirable limitations. An all-tungsten target is freefrom these limitations, but the high specific gravity of tungstenrestricts the size of the target for mechanical reasons.

In accordance with my invention I have provided a target having agreater heat dissipating capacity than an all-tungsten target of thesame weight, and which may be heated to a temperature of brightincandegcence in a pure electron discharge X-ray tu. e.

My new target consists of tungsten and molybdenum as illustrated in theaccompanying drawing in which- Figure 1 is a perspective view of thetarget, and Fig. 2 is a sectional view.

The main body 1 of the target consists of molybdenum and the inset 2consists of tungsten, both metals preferably being in the ductile state.The tungsten inset preferably should be accurately fitted, as bygrinding, into the recess in the molybdenum, so as to get the metalsinto good heat conducting relations. The target should be so arranged inthe tube that the cathode rays are made to focus upon the tungsteninset. The specific gravity of molybdenum is only about one-half that oftungsten and the specific heat of molybdenum is more than twice that oftungsten. Hence this new form of target has nearly double the surfaceand more than double the thermal capacity of a tungsten target of thesame weight. The refractory nature of molybdenum enables the thoroughremoval of gas from the target. When used in an X-ray tube operatingwith a pure electron discharge substantially independent of positiveionization, the present form of target may be heated to brightincandescence without disturbing the normal operation of the tube.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is,

1. An X-ray target comprising a body of molybdenum and a body oftungsten upon which cathode rays may impinge.

2. In an X-ray tube operating with a pure electron discharge, a targetcomprising a body of molybdenum and a body of tungsten in good heatconducting relation thereto.

3. An X-ray target comprising a body of molybdenum and a body oftungsten mechanically fitted therein at the working face, said body ofmolybdenum having a substantially larger heat storage capacity than 9said body of tungsten.

In witness whereof, I have hereunto set my hand this 9th day of April,1915.

WILLIAM D. COOLIDGE.

